Both will and be going to express the idea of the future, and it is rarely incorrect to use either one.

There are, however, differences that might not seem important, but become more so as you become more fluent in the language. These differences are briefly explained below. If you prefer a more detailed explanation, go here.

Note: If you are not sure which one to use, choose "be going to." It is used for the future more often than "will."

Except for undisputed facts, those we cannot change, ("I will be eighteen tomorrow"),will is usually not as definite as be going to.

We use will when we 1) offer to do something, often spontaneously, or 2) we want something to happen, or 3) we are trying to make a decision.

I'll take you to the station. ( = offer)I'll get you there in time to catch your train. ( = want, desire)

That's a good idea. We won't take the car. We'll fly. ( = the act of making a decision)

We use be going towhen we are more sure about something we have decided or that we know from experience. It expresses 1) an intention, when we have already made a decision, or 2) when we know something is going to happen based on what we see in the present.

Look at those black clouds. It's going to rain. I hope you brought your umbrella. ( = The clouds indicate that it's going to rain.)